The potential for nanoparticle-based drug delivery to the brain: overcoming the blood–brain barrier

The development of blood–brain barrier (BBB)-targeting technologies is a very active field of research: targeting therapeutic actives to the central nervous system by means of systemic administration means crossing the BBB, and this is now one of the most challenging problems in drug development. The BBB is a unique regulatory system that protects the brain environment by separating it from direct contact with the circulating blood. In doing so, it impedes at the same time the access of a large number of diagnostic and therapeutic agents into the brain parenchyma. One of the possibilities of bypassing this barrier relies on specific properties of nanoparticulate vectors designed to interact with BBB-forming cells at a molecular level, as a result of which the transport of drugs or other molecules (such as nucleic acids, proteins or imaging agents) could be achieved without interfering with the normal function of the brain. This article summarises several recent example applications, presents emerging work and highlights the directions for further developments in this area.     

Targeted delivery across the blood–brain barrier

The safest and most effective way of targeting drugs to the entire brain is via delivery systems directed at endogenous receptor-mediated uptake mechanisms present at the cerebral capillaries. Such systems have been shown to be effective in animal models including primates, but no clinical trials have been performed so far. This review focuses on the well-characterised transferrin and insulin receptor-targeted systems, as well as on the more recently described systems that use the low-density lipoprotein-related protein 1 receptor, the low-density lipoprotein-related protein 2 receptor (also known as megalin and glycoprotein 330) or the diphtheria toxin receptor (which is the membrane-bound precursor of heparin-binding epidermal growth factor-like growth factor). The possibilities and limitations of these systems are compared and their future for human application is discussed.     

Nano-enabled delivery systems across the blood–brain barrier

The development of drugs to treat disorders of the central nervous system (CNS) faces difficulties in achieving penetration of a drug through the blood–brain barrier (BBB) and allowing the drug to reach its intended target in the brain. There have been strategies to improve drug delivery to the brain through endogenous transport pathways such as passive diffusion, endocytosis, and active transport. Among various strategies, nano-enabled delivery systems offer a promising solution to improve the uptake and targeted delivery of drugs into the brain. Various nanocarriers including liposomes, bolaamphiphiles and nanoparticles can be used as a means to encapsulate drugs, either alone or in combination with targeting ligands. Moreover, most of materials used in nanocarrier fabrication are both biodegradable and biocompatible, thereby increasing the clinical utility of them. Here, we review the possibility to employ nano-enabled materials for delivery of drug across the BBB and the recent advances in nanotechnologies for therapy of the CNS diseases.     

ABC transporters at the blood–brain barrier

Introduction: The blood–brain barrier (BBB) possesses an outstanding ability to protect the brain against xenobiotics and potentially poisonous metabolites. Owing to this, ATP binding cassette (ABC) export proteins have garnered significant interest in the research community. These transport proteins are predominantly localized to the luminal membrane of brain microvessels, where they recognize a wide range of different substrates and transport them back into the blood circulation.

Areas covered: This review summarizes recent findings on these transport proteins, including their expression in the endothelial cell membrane and their substrate recognition. Signaling cascades underlying the expression and function of these proteins will be discussed as well as their role in diseases such as Alzheimer’s disease, epilepsy, amyotrophic lateral sclerosis and brain tumors.

Expert opinion: ABC transporters represent an integral part of the human transportome and are of particular interest at the blood–brain barrier they as they significantly contribute to brain homeostasis. In addition, they appear to be involved in myriad CNS diseases. Therefore studying their mechanisms of action as well as their signaling cascades and responses to internal and external stimuli will help us understand the pathogenesis of these diseases.     

Direct nose to brain drug delivery via integrated nerve pathways bypassing the blood–brain barrier: an excellent platform for brain targeting

Introduction: The blood–brain barrier (BBB) represents a stringent barrier for delivery of neurotherapeutics in vivo. An attempt to overcome this barrier is represented by the direct transport of drugs from the nose to the brain along the olfactory and trigeminal nerve pathways. These nerve pathways initiate in the nasal cavity at olfactory neuroepithelium and terminate in the brain. An enormous range of neurotherapeutics, both macromolecules and low molecular weight drugs, can be delivered to the central nervous system (CNS) via this route.

Areas covered: Present review highlights the literature on the anatomy-physiology of the nasal cavity, pathways and mechanisms of neurotherapeutic transport across nasal epithelium and their biofate and various strategies to enhance direct nose to brain drug delivery. The authors also emphasize a variety of drug molecules and carrier systems delivered via this route for treating CNS disorders. Patents related to direct nose to brain drug delivery systems have also been listed.

Expert opinion: Direct nose to brain drug delivery system is a practical, safe, non-invasive and convenient form of formulation strategy and could be viewed as an excellent alternative approach to conventional dosage forms. Existence of a direct transport route from the nasal cavity to the brain, bypassing the BBB, would offer an exciting mode of delivering neurotherapeutic agents.     

Ultrasound-mediated blood–brain barrier disruption for targeted drug delivery in the central nervous system

The physiology of the vasculature in the central nervous system (CNS), which includes the blood–brain barrier (BBB) and other factors, complicates the delivery of most drugs to the brain. Different methods have been used to bypass the BBB, but they have limitations such as being invasive, non-targeted or requiring the formulation of new drugs. Focused ultrasound (FUS), when combined with circulating microbubbles, is a noninvasive method to locally and transiently disrupt the BBB at discrete targets. This review provides insight on the current status of this unique drug delivery technique, experience in preclinical models, and potential for clinical translation. If translated to humans, this method would offer a flexible means to target therapeutics to desired points or volumes in the brain, and enable the whole arsenal of drugs in the CNS that are currently prevented by the BBB.     

Drug delivery across the blood–brain barrier using focused ultrasound

Introduction: The presence of the blood–brain barrier (BBB) is a significant impediment to the delivery of therapeutic agents to the brain for treatment of brain diseases. Focused ultrasound (FUS) has been developed as a noninvasive method for transiently increasing the permeability of the BBB to promote drug delivery to targeted regions of the brain.

Areas covered: The present review briefly compares the methods used to promote drug delivery to the brain and describes the benefits and limitations of FUS technology. We summarize the experimental data which shows that FUS, combined with intravascular microbubbles, increases therapeutic agent delivery into the brain leading to significant reductions in pathology in preclinical models of disease. The potential for translation of this technology to the clinic is also discussed.

Expert opinion: The introduction of magnetic resonance imaging guidance and intravascular administration of microbubbles to FUS treatments permits the consistent, transient and targeted opening of the BBB. The development of feedback systems and real-time monitoring techniques improve the safety of BBB opening. Successful clinical translation of FUS has the potential to revolutionize the treatment of brain disease resulting in effective, less-invasive treatments without the need for expensive drug development.     

Nano carriers for drug transport across the blood–brain barrier

Effective therapy lies in achieving a therapeutic amount of drug to the proper site in the body and then maintaining the desired drug concentration for a sufficient time interval to be clinically effective for treatment. The blood–brain barrier (BBB) hinders most drugs from entering the central nervous system (CNS) from the blood stream, leading to the difficulty of delivering drugs to the brain via the circulatory system for the treatment, diagnosis and prevention of brain diseases. Several brain drug delivery approaches have been developed, such as intracerebral and intracerebroventricular administration, intranasal delivery and blood-to-brain delivery, as a result of transient BBB disruption induced by biological, chemical or physical stimuli such as zonula occludens toxin, mannitol, magnetic heating and ultrasound, but these approaches showed disadvantages of being dangerous, high cost and unsuitability for most brain diseases and drugs. The strategy of vector-mediated blood-to-brain delivery, which involves improving BBB permeability of the drug–carrier conjugate, can minimize side effects, such as being submicrometre objects that behave as a whole unit in terms of their transport and properties, nanomaterials, are promising carrier vehicles for direct drug transport across the intact BBB as a result of their potential to enter the brain capillary endothelial cells by means of normal endocytosis and transcytosis due to their small size, as well as their possibility of being functionalized with multiple copies of the drug molecule of interest. This review provids a concise discussion of nano carriers for drug transport across the intact BBB, various forms of nanomaterials including inorganic/solid lipid/polymeric nanoparticles, nanoemulsions, quantum dots, nanogels, liposomes, micelles, dendrimers, polymersomes and exosomes are critically evaluated, their mechanisms for drug transport across the BBB are reviewed, and the future directions of this area are fully discussed.     

Efflux proteins at the blood–brain barrier: review and bioinformatics analysis

1.?Efflux proteins at the blood–brain barrier provide a mechanism for export of waste products of normal metabolism from the brain and help to maintain brain homeostasis. They also prevent entry into the brain of a wide range of potentially harmful compounds such as drugs and xenobiotics.

2.?Conversely, efflux proteins also hinder delivery of therapeutic drugs to the brain and central nervous system used to treat brain tumours and neurological disorders. For bypassing efflux proteins, a comprehensive understanding of their structures, functions and molecular mechanisms is necessary, along with new strategies and technologies for delivery of drugs across the blood–brain barrier.

3.?We review efflux proteins at the blood–brain barrier, classified as either ATP-binding cassette (ABC) transporters (P-gp, BCRP, MRPs) or solute carrier (SLC) transporters (OATP1A2, OATP1A4, OATP1C1, OATP2B1, OAT3, EAATs, PMAT/hENT4 and MATE1).

4.?This includes information about substrate and inhibitor specificity, structural organisation and mechanism, membrane localisation, regulation of expression and activity, effects of diseases and conditions and the principal technique used for in vivo analysis of efflux protein activity: positron emission tomography (PET).

5.?We also performed analyses of evolutionary relationships, membrane topologies and amino acid compositions of the proteins, and linked these to structure and function.     

Strategies to assess blood–brain barrier penetration

Background: The principles and screening strategies for brain penetration in drug discovery are important in identifying drug candidates with desirable CNS properties. Objective: Define key variables and assays that are essential for determining brain penetration. Methods: This review covers issues, methods, and strategies for assessing brain penetration of small molecules in drug discovery. Results/conclusion: Brain penetration is assessed using both initial rate and extent at steady-state. Unbound drug is the active species that exerts pharmacological effects. Low brain penetration can be due to low blood–brain barrier (BBB) permeability, P-glycoprotein (Pgp) efflux, or high plasma protein binding. Successful methods include: parallel artificial membrane permeability assay (PAMPA)-BBB permeability, MDR1-MDCKII for Pgp efflux, B-P dialysis for fraction unbound, and in vivo B/P ratio to extrapolate unbound brain drug concentration.     

Nanoparticle-mediated delivery of oligonucleotides to the blood–brain barrier: in vitro and in situ brain perfusion studies on the uptake mechanisms

Abstract

Except for the few exceptions where topical administration is feasible, progress towards broad clinical application of nucleic acid therapeutics requires development of effective systemic delivery strategies. The central nervous system represents a particularly difficult organ for systemic delivery due to the blood–brain barrier. We previously reported a nanoparticulate delivery system for targeted brain delivery of oligonucleotides upon systemic administration, i.e. liposome-encapsulated polyethylenimine/oligonucleotides polyplexes. In this study, cellular uptake and intracellular trafficking of the nanoparticles were further investigated using in situ brain perfusion technique followed by colocalization and fluorescence resonance energy transfer techniques. The brain endothelial uptake and possibly parenchymal accumulation were readily visualized upon administration via internal carotid artery perfusion. The nanoparticles were colocalized with early-endosome antigen, which confirms the brain endothelial uptake through transferrin receptor-mediated endocytosis. Fluorescence resonance energy transfer analysis also suggested the nanoparticles entered the brain endothelial cells while maintaining their integrity. Together, the enhanced brain uptake, as claimed previously, of the antibody-targeted nanoparticles was clearly confirmed with more convincing evidences. In addition, the experimental techniques described here should be applicable to the studies involving nanoparticle-mediated brain delivery of nucleic acid therapeutics.     

Strategies for therapy of retinal diseases using systemic drug delivery: relevance of transporters at the blood–retinal barrier

Introduction: There is an increasing need for managing rapidly progressing retinal diseases because of the potential loss of vision. Although systemic drug administration is one possible route for treating retinal diseases, retinal transfer of therapeutic drugs from the circulating blood is strictly regulated by the blood–retinal barrier (BRB).

Areas covered: This review discusses the constraints and challenges of drug delivery to the retina. In addition, this article discusses the properties of drugs and the conditions of the BRB that affect drug permeability. The reader will gain insights into the strategies for developing therapeutic drugs that are able to cross the BRB for treating retinal diseases. Further, the reader will gain insights into the role of BRB physiology including barrier functions, and the effect of influx and efflux transporters on retinal drug delivery.

Expert opinion: When designing and selecting optimal drug candidates, it's important to consider the fact that they should be recognized by influx transporters and that efflux transporters at the BRB should be avoided. Although lipophilic cationic drugs are known to be transported to the brain across the blood–brain barrier, verapamil transport to the retina is substantially higher than to the brain. Therefore, lipophilic cationic drugs do have a great ability to increase influx transport across the BRB.     

Targeted delivery of antibody-based therapeutic and imaging agents to CNS tumors: crossing the blood–brain barrier divide

Introduction: Brain tumors are inherently difficult to treat in large part due to the cellular blood–brain barriers (BBBs) that limit the delivery of therapeutics to the tumor tissue from the systemic circulation. Virtually no large molecules, including antibody-based proteins, can penetrate the BBB. With antibodies fast becoming attractive ligands for highly specific molecular targeting to tumor antigens, a variety of methods are being investigated to enhance the access of these agents to intracranial tumors for imaging or therapeutic applications.

Areas covered: This review describes the characteristics of the BBB and the vasculature in brain tumors, described as the blood–brain tumor barrier (BBTB). Antibodies targeted to molecular markers of central nervous system (CNS) tumors will be highlighted, and current strategies for enhancing the delivery of antibodies across these cellular barriers into the brain parenchyma to the tumor will be discussed. Noninvasive imaging approaches to assess BBB/BBTB permeability and/or antibody targeting will be presented as a means of guiding the optimal delivery of targeted agents to brain tumors.

Expert opinion: Preclinical and clinical studies highlight the potential of several approaches in increasing brain tumor delivery across the BBB divide. However, each carries its own risks and challenges. There is tremendous potential in using neuroimaging strategies to assist in understanding and defining the challenges to translating and optimizing molecularly targeted antibody delivery to CNS tumors to improve clinical outcomes.     

Lipid-based nanocarrier efficiently delivers highly water soluble drug across the blood–brain barrier into brain

Delivering highly water soluble drugs across blood–brain barrier (BBB) is a crucial challenge for the formulation scientists. A successful therapeutic intervention by developing a suitable drug delivery system may revolutionize treatment across BBB. Efforts were given here to unravel the capability of a newly developed fatty acid combination (stearic acid:oleic acid:palmitic acid?=?8.08:4.13:1) (ML) as fundamental component of nanocarrier to deliver highly water soluble zidovudine (AZT) as a model drug into brain across BBB. A comparison was made with an experimentally developed standard phospholipid-based nanocarrier containing AZT. Both the formulations had nanosize spherical unilamellar vesicular structure with highly negative zeta potential along with sustained drug release profiles. Gamma scintigraphic images showed both the radiolabeled formulations successfully crossed BBB, but longer retention in brain was observed for ML-based formulation (MGF) as compared to soya lecithin (SL)-based drug carrier (SYF). Plasma and brain pharmacokinetic data showed less clearance, prolonged residence time, more bioavailability and sustained release of AZT from MGF in rats compared to those data of the rats treated with SYF/AZT suspension. Thus, ML may be utilized to successfully develop drug nanocarrier to deliver drug into brain across BBB, in a sustained manner for a prolong period of time and may provide an effective therapeutic strategy for many diseases of brain. Further, many anti-HIV drugs cannot cross BBB sufficiently. Hence, the developed formulation may be a suitable option to carry those drugs into brain for better therapeutic management of HIV.     

Murine in vitro model of the blood–brain barrier for evaluating drug transport

In vitro blood–brain barrier (BBB) models help predict brain uptake of potential central nervous system drug candidates. Current in vitro models are composed of brain microvascular endothelial cells (BMEC) that are isolated from rat, bovine, or porcine. However, most in vivo studies on drug transport through the BBB are performed in small laboratory animals, specially mouse and thus murine in vitro BBB models serve as better surrogates to correlate with these studies. Here we describe the functional characterization of a reproducible in vitro model composed of murine BMEC co-cultured with rat primary astrocytes in the presence of biochemical inducing agents. The co-cultures presented high TEER and low sodium fluorescein permeability. Expression of specific BBB tight junction proteins (occludin, claudin-5, ZO-1) and the functionality of transporters (Pgp, GLUT1) were detected by immunocytochemistry and Western blotting. These results indicated a 2.5-fold increase in the expression levels of these proteins in the presence of astrocytes. In addition, a high correlation coefficient (0.98) was obtained between the permeability of a series of hydrophobic and hydrophilic drugs and their corresponding in vivo values. These results together establish the utility of this murine model for future drug transport, pathological, and pharmacological characterizations of the BBB.     

Glutathione PEGylated liposomes: pharmacokinetics and delivery of cargo across the blood–brain barrier in rats

Abstract

Partly due to poor blood–brain barrier drug penetration the treatment options for many brain diseases are limited. To safely enhance drug delivery to the brain, glutathione PEGylated liposomes (G-Technology®) were developed. In this study, in rats, we compared the pharmacokinetics and organ distribution of GSH-PEG liposomes using an autoquenched fluorescent tracer after intraperitoneal administration and intravenous administration. Although the appearance of liposomes in the circulation was much slower after intraperitoneal administration, comparable maximum levels of long circulating liposomes were found between 4 and 24?h after injection. Furthermore, 24?h after injection a similar tissue distribution was found. To investigate the effect of GSH coating on brain delivery in vitro uptake studies in rat brain endothelial cells (RBE4) and an in vivo brain microdialysis study in rats were used. Significantly more fluorescent tracer was found in RBE4 cell homogenates incubated with GSH-PEG liposomes compared to non-targeted PEG liposomes (1.8-fold, p?<?0.001). In the microdialysis study 4-fold higher (p?<?0.001) brain levels of fluorescent tracer were found after intravenous injection of GSH-PEG liposomes compared with PEG control liposomes. The results support further investigation into the versatility of GSH-PEG liposomes for enhanced drug delivery to the brain within a tolerable therapeutic window.     

Ibogaine labeling with 99mTc-tricarbonyl: Synthesis and transport at the mouse blood–brain barrier

The ^99mTc-tricarbonyl core may be used as an ideal tool for gamma-labeling ligands in noninvasive SPECT imaging. However, most ^99mTc-tricarbonyl-labeled agents have difficulty crossing the blood–brain barrier (BBB). We radiolabeled the neuroactive indole ibogaine with ^99mTc-tricarbonyl and measured its transport into the mouse brain by in situ brain perfusion. We measured the interactions of [^99mTc(CO)₃-ibogaine]⁺ and ^99mTc-tricarbonyl with the main BBB efflux transporters P-gp and BCRP in vitro and in vivo. Ibogaine was radiolabeled (yield: over 95%). [^99mTc(CO)₃-ibogaine]⁺ entered the brain (K_in) poorly (0.18 µL/g/s), at about the same rate as ^99mTc-tricarbonyl (0.16 µL/g/s) and [^99mTc-sestamibi]⁺ (0.10 µL/g/s). The CNS tracer [^99mTc-HMPAO]⁰ entered the brain ～70-times higher than [^99mTc(CO)₃-ibogaine]⁺. In vitro studies revealed that neither [^99mTc(CO)₃-ibogaine]⁺ nor ^99mTc-tricarbonyl ion were substrates for P-gp or BCRP. But lowering the membrane dipole potential barrier with phloretin enhanced the brain transport of [^99mTc(OH₂)₃(CO)₃]⁺ ～3-fold. Thus, ibogaine directly labeled with ^99mTc-tricarbonyl is not suitable for CNS imaging because of its poor uptake. Brain transport is not restricted by efflux transporters but is reduced by its lipophilicity and interaction with the membrane-positive dipole potential.